The explosive craft was found on Friday afternoon in the Baltic on the same latitude as the southern cape of Öland, south of Gotland. Its origin is unknown.
"Our assessment is that it is a disposable vessel with a small bomb, used to clear mines. It is not likely that the explosives will detonate,” said Johannes Hellqvist, information officer at the Swedish Armed Forces to public broadcaster Swedish SVT.
Nevertheless, the Swedish Maritime Administration has issued a navigation warning about a "dangerous object”, advising ships to keep a security distance of three nautical miles (5.5 km) around the vessel.
The military refused to speculate in whether it could have be placed there by terrorists. More likely, there are historical explanations for its whereabouts, Johannes Hellqvist told SVT: "This part of the Baltic has a whole lot of mines from World War II. It could have drifted away, and would then be here for perfectly natural reasons."
The vessel is constructed so that its built-in explosives are supposed to detonate when approaching a mine to disarm it, at the same time destroying itself.
Since the sub was found outside of Sweden's territorial waters, the Swedish Armed Forces are not responsible for disarming it. They will discuss its future on Monday.